Various types of methods and devices have been proposed for storing grains such as unpolished rice or the like over a long period of time.
The method for packing rice for long-term storage disclosed in Japanese Patent Application Laid-Open No. 48-26516 proposes means for placing rice within a bag and substituting the air within the bag with carbon dioxide gas, sealing the bag with the carbon dioxide gas at a ratio of 0.8 to 0.9 apparent specific gravity within the bag, and drying the rice until there is a low moisture content with the moisture content of the rice and the moisture content of the carbon dioxide gas in equilibrium. However, in this storage means, the principal object is only the moisture content of the rice, and no consideration whatsoever is given to insecticidal action against grain weevils included in the rice. Therefore, the carbon dioxide gas passes through the bag and is dispersed, and in accordance with the decrease in the concentration of the carbon dioxide gas, grain weevils propagate.
Further, Japanese Patent Application Laid-Open No. 49-116246 discloses a method for storing grains or the like in an inert gas atmosphere. It is proposed that grains are filled up in a silo which has an inert gas atmosphere, and are stored for a long time by the pressure, temperature and humidity within the storage tank being adjusted automatically. However, in this invention, the silo, which is a strong structure, is needed. Further, because the silo must be built air-tight, it is impossible to transport. A completely sealed, large equipment is needed, and the manufacturing cost thereof is also expensive.
A method of exterminating harmful insects in stored unpolished rice is disclosed in Japanese Patent Application Laid-Open No. 1-279812. This invention discloses exterminating harmful insects in unpolished rice by creating an atmosphere having a vaporized carbon dioxide gas concentration of 50% or more. However, a large silo which is a strong structure is presupposed as the insecticidal device, and the concentration of the carbon dioxide gas filled therein is extremely great. Therefore, a means for reducing the amount of carbon dioxide gas used is also disclosed.